It is well known to those skilled in the art that the most accurate way to install tension into fasteners in a joint is to control tightening by developing an indication of the tension in the fastener either directly from the fastener or another component used with the fastener. Fastener installation based on torque control can result in wide variations in the tension in the fastener because of variations in fastener lubricity, burrs on the thread of threaded fasteners, and variations in the friction at the surface of the joint component against which the bearing surface of a threaded fastener bears.
U.S. Pat. No. 3,187,621 is directed to a pre-load indicating washer adapted for use with a threaded fastener and intended to develop an indication of proper tensioning of the fastener. This pre-load indicating washer has a plurality of protuberances which are struck and partially sheared from an annular body to leave indentations in the annular body. With this pre-load indicating washer in a joint, the protuberances are pushed back into the indentations in the annular body as the joint is tightened and tightening is stopped when the protuberances have been "collapsed" back into the indentations in the annular body a prescribed amount. This condition, typically, is sensed by a "feeler" gage inserted into a gap in the joint. U.S. Pat. No. 5,015,132, U.S. Pat. No. 5,370,483, and U.S. Pat. No. 5,487,632 also are directed to this type of pre-load indicating washer.
If the protuberances of this pre-load indicating washer extend beyond the bearing surface of the fastener with which it is used, as is the case with straight-sided protuberances and fasteners having comparatively smaller bearing surfaces, the protuberances will not be collapsed back into the indentations in the annular body of the washer properly. Instead of the load being applied over the entire lengths of the protuberances, it is applied only over those portions of the protuberances which are in contact with the bearing surface of the fastener. As a result, some portions of the protuberances are not collapsed back into the indentations leading to erroneous indications of the tension of the fastener. The use of such pre-load indicating washers with fasteners having comparatively smaller bearing surfaces has not been practical.